Lee Hart wrote:
> Bill Dube wrote:
>>> Put it very close to the heating element. If you put the thermostat
>>> any distance away, the system will "hunt" and the temperature can
>>> swing wildly.
>
> Martin WINLOW wrote:
>> If you put it close to the wire then the thermostat will be
>> constantly switching on and off without actually heating anything,
>> wearing out the relay contacts unnecessarily. If you put it halfway
>> up the pack by the time the thermostat has got warm at least some
>> heating effect will have occurred... A bit of trial and error will be
>> required...
>
> Exactly so. I designed HVAC systems for Robertshaw and Honeywell for
> many years, and can provide some insight.
>
> When the heater control is just on/off, and the temperature sensor
> is a
> thermostat that also switches on/off, it is called a "bang-bang"
> system.
> It is the simplest kind of control, but adequate when you don't need
> precise temperature control.
>
> With such a system, choose the distance between the heater and
> temperature sensor so it cycles on/off a few times per hour. If the
> sensor is too close, it cycles too fast, which wears things out
> quicker.
> If the sensor is too far away, it cycles too slowly, leading to large
> temperature extremes.
>
>> I'd be nice to have at least 9 temp probes on a 3 x 3 grid throughout
>> the pack to monitor how the heat moves with time.
>
> There's nothing wrong with using many bimetal temperature sensing
> switches. They are pretty small and cheap. If they open when hot, wire
> them in series; any one turning off turns off the heater.
>
>> In my fairly well insulated battery box, the temp stays fairly
>> constant with the aid of an electric blanket underneath the cells
>> (separated by a piece of 9mm plywood).
>
> You want all the cells/batteries at the same temperature. One way is
> to
> put them all in the same box, tight against each other, so heat can
> flow
> freely from one to the others.
>
> If they have to be in separate boxes, or aren't tightly touching each
> other, you're likely to need separate heaters and thermostats for each
> group.
>
> I wouldn't use wood in a battery box; it's a poor insulator, conducts
> when wet, burns, rots if it gets battery acid on it, etc.
>
> Also be careful about putting any insulation around the wire. The
> heater
> will work poorly, or overheat, melt, or even start a fire.
>
> If the wire is the kind that automatically changes its resistance with
> temperature, then it will stop producing heat if you insulate it.
>
> If the wire is the more normal kind that draws pretty much the same
> current regardless of its temperature, then it superheats if you
> surround it with insulation.
>
> I make battery heaters by cutting a thin aluminum plate about the size
> of the bottom of the battery box. I tape the resistance wire to it
> with
> double-sided tape, spreading it out so the strands are about 2" from
> each other. I then put it in a polyethylene bag, pour in potting
> compound, and use a vacuum cleaner to suck out the excess air. When it
> cures, I have a waterproof insulated heating plate.
>
> I put this directly under the batteries, aluminum side up. The
> aluminum
> "heat sinks" the wire so it won't melt, spreads the heat out over the
> batteries, and provides a large surface area to transfer the heat to
> the
> batteries. I put a sheet of styrafoam insulation under the sheet. It
> provides thermal insulation, and its softness prevents pinching the
> wire
> insulation.
>
> The aluminum sheet has a ground wire, so if the potting or baggie
> leaks,
> I can detect it via a GFCI.
> --
> Lee A. Hart | Ring the bells that still can ring
> 814 8th Ave N | Forget the perfect offering
> Sartell MN 56377 | There is a crack in everything
> leeahart earthlink.net | That's how the light gets in -- Leonard Cohen
>
>
Hi Lee,
Your installation sounds very professional as I would expect.
However, I can't help but plug the common or garden domestic electric
blanket - so I'm going to do it again. Your last post raises some
important issues which I think are readily addressed by using an
electric blanket but without any significant cost or hassle.
1/ I too have my blanket sitting on an inch of polyurethane (foil-
backed) insulation which - as you note - offers a more gentle base
than something hard. Done again, I would use foil-backed bubble-wrap
type insulation all round as I have used it on my box lid and found it
extremely effective. Initially, I was concerned about its proximity
to the shunt resistors but have found no problems - thus far (after
about 3k miles). But I have an audible/texting-based smoke alarm
installed - just in case. I have a new BMMS system in the works which
moves the shunts away from the cells too, so this wont be an issue any
more.
2/ The plywood layer above the blanket protects it from the bottoms of
the cells and, as you note, offers little thermal resistance - ideal
then!
3/ The blanket comes with at least 3 important safety factors built-in
- designed to protect a much more valuable 'payload' than a bunch of
TS cells, ie a sleeping bod... - a/ it is isolated from the mains, b/
it has automatic built-in thermal control and c/ it has built in
element failure protection. I use a standard household digital
thermostat to control the heating and it is on whenever the mains is
connected to the vehicle.
4/ Lastly, it is as cheap as chips (that's french fries to you).
I think you would be hard pressed to find a device designed for an
entirely different application that lends itself so well to EV use.
Regards, Martin Winlow
Herts, UK
http://www.evalbum.com/2092
www.winlow.co.uk
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